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Unit 3: Plant Taxonomy and Classification

Introduction to Taxanomy:

Taxanomy is basically concerned with the classification of organisms.Before classifying

organisms it is necessary to identify and name them. A particular group of individuals ,

unique in several respects is assigned to a species which are then grouped in to genera,

families, orders etc. Taxanomy was recognized as a formal subject only in 1813 by A. P.

decandolle as a combination of two Greek words taxis ( arrangement) and nomos (rules or

laws) in his famous work Theorie Elementaire De La Botanique.

Taxanomy is defined as the science dealing with the study of classification, including its

bases, principles, rules and procedures. (Davis &Heywood 1963). And systematics as a

scientific study of the kinds and diversity of organisms,and all relationships between them. A

broader definition of taxanomy to coincide with systematic recognizes it as the study and

description of variation in organisms, the investigation of causes and consequences of this

variation, and the manipulation of the data obtained to produce a system of classification.

Taxanomy Components: Taxanomy has four basic components namely

a) Identification b) Description c) Nomenclature d) Classification

Types of Classification:

Classification is the arrangement of organisms into groups on the basis of similarities. These

groups are in turn assembled into more inclusive groups, until all organisms have been

assembled in to the single most inclusive group.

Broadly speaking there are three main classification systems;

Artificial Classification: This system of classification is based on arbitrary easily observable

characters such as habit, colour, number, form, or other similar features. These classifications

remained dominant from 300 B.C. up to about 1830. The earliest system of artificial

classification was proposed by Theophrastus who classified plants as herbs, under shrubs,

shrubs and trees on the basis of habit. Other advocates of artificial system of classification are

Secundus, Discordes etc.

Carolus Linneous classification is considered as the best of artificial systems which

dominated for over 75 years and finally replaced by natural system of classification.LIinneous

recognized 24 clases on the basis of number, size and union of stamens.

Natural classification: This system of classification is based on natural affinities or overall

similarities. These systems started with M. Adanson & culminated with Bentham and Hooker.

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Natural systems of 18th

and 19th

centuries used morphology as criteria for overall similarity,

Nowadays overall similarity is judged on the basis of features derived from all the available

fields of taxanomic information.

Some important natural systems of classification are: Adanson, A. L. dejusseu, A. P.

decandolle, and Bentham and Hooker.

Phylogenetic classification: This system is based on evolutionary descent of a group of

organisms, the relationship depicted either through a phylogram or a cladogram.Most of the

propounders of these systems of classification have emphasized on certain pre selected

characters which are considered to be of phylogenetic importance. The most widely known

phylogenetic systems are those of Engler & Prantl, Hutchinson, Takhtajan etc.

Bentham &Hooker’s System of Classification:

George Bentham (1800- 1884) and Joseph Dalton Hooker ( 1817- 1911) worked together to

bring out a Genera Plantarum (1862-1883) where in they presented their outstanding system

of classification. The first part of Genera Plantarum appeared in july 1862 and the last part in

April 1883. This system of classification was based on that of de Candolle but it was a

refinement of the latter.This system was widely accepted in Britain and Commonwealth

countries but did not find a ground in Europe and America where Englerian system was

preferred.

A synopsis of Bentham and Hookers classification

Orders Genera Species

Polypetalae

82 2610 31874

Gamopatalae 45 2619 34556

Monochlamydae 36 801 11784

Gymnosperms 3 44 415

Monocotyledons 34 1495 18576

Total 200 7569 97205

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The following is the summary of Bentham & Hooker‟s classification

1.Polypetalae (corolla of distinct petals)

Series i. Thalamiflorae (petals and stamens hypogynous disc absent.) It

includes six orders: Ranales, caryophyllineae, parietals, guttiferales,polygalineae,

malvales.

Series ii. Discifloral (petals and stamens hypogynous and a nectiferous disc surrounds the

base of the ovary). It contains following

orders: Geranials Calastrales Olacales Sapinales

Series iii. Calycifloral (petals and stamens perigynous or sometimes epigynous.It

includes five orders: Ficoidales, myrtales ,umberalles , passiflorals

2.Gamopetalae (petals of corolla are of partially or completely fused)

Series i. Inferae (ovary inferior).It includes three orders Rubiales Companulaceae

Asterales

Series ii. Heteromerae (ovary superial stamens as many or twice as many as corolla lobes

carpels more than two). It has three orders Ericals , Ebenals Primulales.

Series iii. Bicarpellatae (ovary superior, stamens as many as the corolla lobes or fewer

carprels usually two).

3.Monochlamydeae (petals absent)

Series i. Curvembryeae (embryo curved round the endosperm ovule curved usually one)

Series ii. Multiovulatae aquaticae (Aquatics with numerous ovule)

Series iii. Multivulatae terrestris (terrestrial plants with numerous ovule)

Series iv. Microembryeae (embryo very small in copious endosperm)

Series v. Daphnales (ovary with one carpel and single ovule)

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Series vi. Achasmydosporeae (ovary usually inferior , unilocular and one to three ovuled.

Series vii Unisexuales (flowers unisexuals).

Series viii Ordines anomaly (the families of uncertain relationship were placed in this

series)

B. Gymnospermae

Series i. Microspermae

Series ii. Epigynae

Series iii. Coronarieae

Series iv . Calycineae

Series v. Nudiflorae

Series vi. Apocaroae

Series vii. Glumaceae.

Angiosperm Phylogeny Group:

The Angiosperm Phylogeny Group, or APG, refers to an informal international group of

systematic botanists who came together to try to establish a consensus on

the taxonomy of flowering plants (angiosperms) that would reflect new knowledge about

plant relationships discovered through phylogenetic studies.

As of 2016, four incremental versions of a classification system have resulted from

this collaboration, published in 1998, 2003, 2009 and 2016. An important motivation for the

group was what they considered deficiencies in prior angiosperm classifications since they

were not based on monophyletic groups (i.e., groups that include all the descendants of a

common ancestor). APG publications are increasingly influential, with a number of major

herbaria changing the arrangement of their collections to match the latest APG system.

Angiosperm classification

In the past, classification systems were typically produced by an individual botanist or by a

small group. The result was a large number of systems . Different systems and their updates

were generally favoured in different countries. Examples are the Engler system in continental

Europe, the Bentham & Hooker system in Britain (particularly influential because it was used

by Kew), the Takhtajan system in the former Soviet Union and countries within its sphere of

influence and the Cronquist system in the United States.

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Before the availability of genetic evidence, the classification of angiosperms (also known

as flowering plants, Angiospermae,Anthophyta or Magnoliophyta) was based on

their morphology (particularly of their flower) and biochemistry (the kinds of chemical

compounds in the plant). After the 1980s, detailed genetic evidence analysed

by phylogenetic methods became available and while confirming or clarifying some

relationships in existing classification systems, it radically changed others. This genetic

evidence created a rapid increase in knowledge that led to many proposed changes; stability

was "rudely shattered". This posed problems for all users of classification systems (including

encyclopaedists). The impetus came from a major molecular study published in 1993 based

on 5000 flowering plants and a photosynthesis gene (rbcL).This produced a number of

surprising results in terms of the relationships between groupings of plants, for instance the

dicotyledons were not supported as a distinct group. At first there was a reluctance to develop

a new system based entirely on a single gene. However, subsequent work continued to

support these findings. These research studies involved an unprecedented collaboration

between a very large number of scientists. Therefore, rather than naming all the individual

contributors a decision was made to adopt the name Angiosperm Phylogeny Group

classification, or APG for short. The first publication under this name was in 1998 and

attracted considerable media attention. The intention was to provide a widely accepted and

more stable point of reference for angiosperm classification.

As of 2016, three revisions have been published, in 2003 (APG II), in 2009 (APG III) and in

2016 (APG IV), each superseding the previous system. Thirteen researchers have been listed

as authors to the three papers, and a further 43 as contributors.

A classification presents a view at a particular point in time, based on a particular state of

research. Independent researchers, including members of the APG, continue to publish their

own views on areas of angiosperm taxonomy. Classifications change, however inconvenient

this is to users. However, the APG publications are increasingly regarded as an authoritative

point of reference and the following are some examples of the influence of the APG system:

i. A significant number of major herbaria, including Kew, are changing the order of their

collections in accordance with APG.

ii. The influential World Checklist of Selected Plant Families (also from Kew) is being

updated to the APG III system.

iii. In the United States in 2006, a photographic survey of the plants of the USA and Canada is

organized according to the APG II system.

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iv. In the UK, the 2010 edition of the standard flora of the British Isles (by Stace) is based on

the APG III system. The previous editions were based on the Cronquist system.

Principles of the APG system

The principles of the APG's approach to classification were set out in the first paper of 1998,

and have remained unchanged in subsequent revisions. Briefly, these are:

The Linnean system of orders and families should be retained. "The family is central

in flowering plant systematics." An ordinal classification of families is proposed as a

"reference tool of broad utility". Orders are considered to be of particular value in teaching

and in studying family relationships.

Groups should be monophyletic (i.e. consist of all descendants of a common ancestor).

The main reason why existing systems are rejected is because they do not have this property,

they are not phylogenetic.

A broad approach is taken to defining the limits of groups such as orders and families.

Thus of orders, it is said that a limited number of larger orders will be more useful. Families

containing only a single genus and orders containing only a single family are avoided where

this is possible without violating the over-riding requirement for monophyly.

Above or parallel to the level of orders and families, the term clades is used more

freely. (Some clades have later been given formal names in a paper associated with the 2009

revision of the APG system. The authors say that it is "not possible, nor is it desirable" to

name all clades in a phylogenetic tree; however, systematists need to agree on names for some

clades, particularly orders and families, to facilitate communication and discussion.

APG I (1998)

The initial 1998 paper by the APG made angiosperms the first large group of organisms to be

systematically re-classified primarily on the basis of genetic characteristics The paper

explained the authors' view that there is a need for a classification system for angiosperms at

the level of families, orders and above, but that existing classifications were "outdated". The

main reason why existing systems were rejected was because they were

not phylogenetic, i.e. not based on strictly monophyletic groups (groups which consist

of all descendants of a common ancestor). An ordinal classification of flowering plant

families was proposed as a "reference tool of broad utility". The broad approach adopted to

defining the limits of orders resulted in the recognition of 40 orders, compared to, for

example, 232 in Takhtajan's 1997 classificationIn 1998 only a handful of families had been

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adequately studied, but the primary aim was to obtain a consensus on the naming of higher

orders. Such a consensus proved relatively easy to achieve but the resultant tree was highly

unresolved. That is, while the relationship of orders was established, their composition was

not.

Other features of the proposed classification included:

Formal, scientific names are not used above the level of order, named clades being

used instead. Thus eudicots andmonocots are not given a formal rank on the grounds that "it is

not yet clear at which level they should be recognized".

A substantial number of taxa whose classification had traditionally been uncertain are

given places, although there still remain 25 families of "uncertain position".

Alternative classifications are provided for some groups, in which a number of

families can either be regarded as separate or can be merged into a single larger family. For

example, the Fumariaceae can either be treated as a separate family or as part

of Papaveraceae.

A major outcome of the classification was the disappearance of the traditional division of the

flowering plants into two groups, monocots and dicots. The monocots were recognized as a

clade, but the dicots were not, with a number of former dicots being placed in separate groups

basal to both monocots and the remaining dicots, the eudicots or 'true dicots' The overall

scheme was relatively simple. This consisted of a grade consisting of isolated taxa (referred to

as ANITA), followed by the major angiosperm radiation, clades of monocots, magnolids and

eudicots. The last being a large clade with smaller subclades and two main

groupings, rosids and asterids, each in turn having two major subclades.

APG II (2003]

As the overall relationship between groups of flowering plants became clearer, the focus

shifted to the family level, in particular those families generally accepted as problematic.

Again, consensus was achieved relatively easily resulting in an updated classification at the

family level The second paper published by the APG in 2003 presented an update to the

original classification of 1998. The authors stated that changes were proposed only when

there was "substantial new evidence" which supported them. The classification continued the

tradition of seeking broad circumscriptions of taxa, for example trying to place small families

containing only one genus in a larger group. The authors stated that they have generally

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accepted the views of specialists, although noting that specialists "nearly always favour

splitting of groups" regarded as too varied in their morphology. APG II continued and indeed

extends the use of alternative 'bracketed' taxa allowing the choice of either a large family or a

number of smaller ones. For example, the large Asparagaceae family includes 7 'bracketed'

families which can either be considered as part of the Asparagaceae or as separate families.

Some of the main changes in APG II were:

New orders are proposed, particularly to accommodate the 'basal clades' left as

families in the first system.

Many of the previously unplaced families are now located within the system.

Several major families are re-structuredIn 2007, a paper was published giving a linear

ordering of the families in APG II, suitable for ordering herbarium specimens, for example

APG III (2009)

The third paper from the APG updates the system described in the 2003 paper. The broad

outline of the system remains unchanged, but the number of previously unplaced families and

genera is significantly reduced. This requires the recognition of both new orders and new

families compared to the previous classification. The number of orders goes up from 45 to 59;

only 10 families are not placed in an order and only two of these

(Apodanthaceae and Cynomoriaceae) are left entirely outside the classification. The authors

say that they have tried to leave long-recognized families unchanged, while merging families

with few genera. They "hope the classification [...] will not need much further change."

A major change is that the paper discontinues the use of 'bracketed' families in favour of

larger, more inclusive families. As a result, the APG III system contains only 415 families,

rather than the 457 of APG II. For example, the agave family (Agavaceae) and the hyacinth

family (Hyacinthaceae) are no longer regarded as distinct from the broader asparagus family

(Asparagaceae). The authors say that alternative circumscriptions, as in APG I and II, are

likely to cause confusion and that major herbaria which are re-arranging their collections in

accordance with the APG approach have all agreed to use the more inclusive families This

approach is being increasingly used in collections in herbaria and botanic gardens. In the same

volume of the journal, two related papers were published. One gives a linear ordering of the

families in APG III; as with the linear ordering published for APG II, this is intended for

ordering herbarium specimens, for example. The other paper gives, for the first time, a

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classification of the families in APG III which uses formal taxonomic ranks; previously only

informal clade names were used above the ordinal level.

APG IV (2016)

In the development of a fourth version there was some controversy over the methodology and

the development of a consensus proved more difficult than in previous iterations. In

particular Peter Stevens questioned the validity of discussions regarding family delimitation in

the absence of changes of phylogenetic relationships.

Further progress was made by the use of large banks of genes, including those

of plastid, mitochondrial and nuclearribosomal origin, such as that of Douglas Soltis and

colleagues (2011). The fourth version was finally published in 2016. It arose from an

international conference hosted at the Royal Botanical Gardens in September 2015 and also

an online survey of botanists and other users. The broad outline of the system remains

unchanged but several new orders are included (Boraginales, Dilleniales, Icacinales,

Metteniusiales and Vahliales), some new families are recognised (Kewaceae,

Macarthuriaceae, Maundiaceae, Mazaceae,Microteaceae, Nyssaceae, Peraceae, Petenaeaceae

and Petiveriaceae) and some previously recognised families are lumped

(Aristolochiaceae now includes Lactoridaceae andHydnoraceae; Restionaceae now re-

includes Anarthriaceae and Centrolepidaceae; and Buxaceae now includesHaptanthaceae).

Due to nomenclatural issues, the family name Asphodelaceae is used instead

of Xanthorrhoeaceae, and Francoaceae is used instead of Melianthaceae (and now also

includes Vivianiaceae). This brings the total number of orders and families recognized in the

APG system to 64 and 416, respectively. Two additional informal major

clades, superrosidsand superasterids, that each comprise the additional orders that are

included in the larger clades dominated by the rosidsand asterids are also included. APG IV

also uses the linear approach (LAPG) as advocated by Haston et al. (2009) In a supplemental

file Byng et al. provide an alphabetical list of families by orders. (Source Wikipedia

(verbatim).

Numerical Taxonomy:

The analysis of various types of taxonomic data by mathematical or computerized methods is

called numerical taxonomy or taximetrics. This approach of systematic involves the numerical

evaluation of similarities or affinities between taxonomic units , and then arrangement of

these units into taxa on the basis of their affinities.

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Adanson (1763), a French botanist, was the first to put forward a plan for assigning

numerical valves on the similarity between organisms .He tried to use as many characters as

possible for the classification ,and such classifications were recognized as Adansonian

classification.

The use of modern electronic data processing techniques have helped in the evolution

of several new classifications of plants during the past 25 years.

Heywood (1967),has defined numerical taxonomy as „‟ the numerical evaluation of

the similarity between groups of organisms and the ordering of these groups into higher

ranking taxa on the basis of these similarities logical steps of numerical taxonomy following

are the successive steps used in construction of taxonomic groups:

i. Operational taxonomic units (OTUS)

ii. Unit taxonomic character or attribute

iii. Coding of characters

vi. Estimation of resemblances

v. Cluster analysis

Operational Taxonomic Units:- The lowest ranking taxa or the basic units in any specific

study are called operational taxonomic unit or OTUs . An OTU may be an individual, a

species, a genus, or a taxon of other higher ranks. Therefore, the rank of OTUs may differ

from study to study .Usually,the species is treated as an OTU .OTU is thus the sample from

which data is collected.

Coding of characters:-Coding of characters makes the use of computers essential as a

huge amount of data is to be generally handled. Here the selected unit characters are assigned

a symbol or mark eg.1,2,3 …. ,or + and -, or 1+,2+,3+ etc.This is called coding of character.

The symbol „NC‟ is used for”no comparision” .

Coding of characters is of the following types:

A. Two state: It is also known as binary coding or presence absence coding. The character in

this type of coding are divided as 1 and 0, or + and - . The characters recorded as 1 or + are

positive while as those recorded as 0 or – are negative NC is used for no comparison.

B. Multistate coding; Each quantative multistate character may be expressed by a single

numerical value such as 1, 2, 3, 4……. Or A ,B, C, D corresponding to range of variation of

the character e,g,

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Flower colour Character state

White A

Red B

Yellow C

Purple D

C) Cluster Analysis:- Clusters are the groups of OTUS, cluster analysis is the method of

arranging OTUS in order of their decreasing similarity with the help of estimation of

resemblances, the affinities of different OTUS are determined. OTUS of similar affinities are

grouped together in different taxa. A taxanomic system is constructed on the basis of

resemblances and differences between groups of OTUS or clusters. The main features of

cluster analysis are arranged in the form of dendrogram.

Cladogram:- Relationship depicting diagrams ae called phylograms.Cladogram is a special

type of phylogram constructed through cladistic methodology. Diagram is constructed with

the premise in mind that all descendants of a common ancestor should be placed in the same

group i.e, group should be monophyletic. If some of the descendents are left out these render

the group as paraphyletic these are brought back into the group to make it monophyletic.

Similarly, if the group is paraphyletic it is split to create monophyletic taxa. The branching in

the diagram is based on the degree of advancement (apomorphy) in the descendants, the

longest branching line represent the most advanced group.

Cladogram is thus a branching diagram of entities where the branching is based on

inferred historical connections between the entities as evidenced by synapomorphies. It is thus

a phylogenitic or historical dendrogram.

Cladogram takes into account the “ Principle of Parsimony” according to which the most

likely evolutionary route is the shortest hypothetical pathway of changes that explains the

pattern under observation.

Phenogram:- is a diagram constructed on the basis of numerical analysis of phonetic data.

Such a diagram is the result of utilization of a large number of characters usually from all

available fields and involve calculating the similarity between taxa and constructing a

diagram through cluster analysis. Such a diagram is very useful, firstly because it is based on

a large number of characters, and secondly because a hierarchial classification can be

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achieved by depicting upon the threshold levels of similarity between taxa assigned to various

ranks.

Cladogram

This is a relationship depicting

diagram.

It is constructed on the basis of cladistic

methodology.

It is a phylogenetic diagram based on

the synapomorphic evidences.

It is based on genealogical descent or

historical evolution.

Cladogram can be represented by

phylogenetic trees eg.Bessey‟s cactus

or Conquirst‟s bubble diagram or

Takhtajan‟s flower diagram.

Phenogram

This is also a relationship depicting diagram.

It is constructed on the basis of numerical

taxonomy or taximetrics.

Cluster analysis forms the basis of

constructing phenogram.

It is based on a large number of characters

derived from many fields of study.

Phenograms can be presentedas contour

diagram.These were originally developed by

Polish phytosociologists under the name of

Wroclow diagram.

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Herbaria and their role:

A collection of dried and pressed plants arranged according to a classification system and

available for study or reference is known as herbarium(Plural herbaria),a name first applied

by Linneaus. Luca Ghini (1490-1556) of Italy has been the initiator of the art of herbaria.

Role of herbaria:Some important roles of herbaria are as under:

1.Modern herbaria are used as reference centers for identification of plants in the fields of

taxonomy,ecology,agriculture,pharmacy etc.

2.modern herbaria are used as documentation centers because they contain specimen of new

taxa ,plants belonging to new discoveries, plants of economic importance,voucher specimens

of cytogenetic studies etc,

3.Genetists,pharmacists ,chemists etc. use herbaria as a data storehouse as they store data on

ecology, habitat,distribution of plants etc.

4.Herbaria act as service institutions as they train researchers,doctors, environmentalist for

studies.5.List of endangered species of any region may be prepared only by herbarium

specimens.

6.Our knowledge of the distribution of plants of any region may be prepared only by

herbarium specimens.

7.Monographs of generas and families are prepared only by herbarium specimens.

8.Herbaria preserve type specimens and thus act as a repository of chromosomes.

9.Herbaria material are used in studying the palynology, anatomy , and chemical aspects of

desired plants.

Important herbaria of world:

1.Museum of natural history Paris………………………….. 6.5 million specimen

2.Royal botanic garden Kew……………………………… over 5 million specimen

3.Komarov botanical institute,Leningrad……………. Over 5 million specimen

4.Conservatory of botanical garden Geneva………….. 4.5 million specimen

5.Harvard university herbaria,Cambridge………………… 4.3 million specimen

Important herbaria of India:

1.The central National Herbarium ,Calcutta………………..2.5 million specimen

2.Herbaria of Forest research institute,Dehradun……… 0.3 million specimen

3.Botanical Survey of India

a. Eastern circle herbarium,Shillong………………… 1 million specimen

b.Southern circle herbarium,Coimbatore…………. 0.175 million specimen

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c.Western circle herbarium,Pune…………………… 0.05 million specimen

d.Northern circle herbarium,Dehradun…………… 0.04 million specimen

e.Central circle herbarium,Allahabad………………0.04 million specimen

4. Blatter herbarium, Bombay……………………… 0.1 million specimen

5. National Botanical Garden herbarium,Lucknow………………….. 0.08 million specimens

Botanical gardens and their roles:

Botanical gardens are the institutions that maintain the living plant collections of

different varieties of plants, including the ornamental and cultivated ones,wild,medicinal,of

economic importance,of various geographical regions,of special interest etc.They are of value

not only to the botanists,horticulturists and foresters but also to millions of people.

A big botanical garden contains plant species from all parts of globe. It also includes

greenhouses,a library,a herbarium,research laboratories, and several miscellaneous resources

including photographs, paintings,illustrations,reprints,notebooks and specimens of several

types.It is ,therefore, not merely a garden but a botanical institution.

Role of botanical gardens:-The following roles are assigned to botanical gardens:

1.Modern botanical gardens serve as centers for documentation ,research,reference,data

storage,education, conservation,and several other biological facilities to mankind.

2.T hey provide information of local flora and are used in the preparation of monographs.

3.Several gardens supply seeds and materials for botanical investigation.

4.They provide information on the protection of endangered species,and propagation of rare

plants.

5.They provide aesthetically pleasing environment for tourism.

Major botanical gardens of the world:

At present there are over 600 botanical gardens in the world.

1.First botanical garden was established by Luca Ghini at Pisa Italy in 1543.

2.Oxford University botanic garden,oxford,England established in 1621.

3.Botanic garden of university of Moscow, Russia established in 1707.

4.Botanic garden ,National Museum of Natural history,Beijing established in 1930

5.The Jardin Botanique de Montreal of Canada established in 1936.

Major botanical gardens of India:

1.Mysore botanical garden, Bangalore established in 1760 by Haider Ali and established as a

real botanical garden in 1856.

2.Lloyd botanic garden ,Darjeeling established in 1910.

3.National botanic garden,Lucknow established in 1946.

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Botanical garden of Forest Research Institute,Dehradun established in 1934.

4.Indian Botanical garden ,Calcutta is the largest and oldest botanical garden of India.